FIG. 1 shows a spacecraft 10 that is 3-axis controlled by the action of strings of thrusters. Two strings of thrusters are employed, an A string and a B string. Thrusters from each string are disposed on an Earth facing panel 12 and an anti-Earth facing panel 14, and are oriented so as to provide full three-axis thrust forces and torques with each string. Full three-axis thrust torques can also be accomplished with a combination of 6 thrusters from both strings. Each thruster string includes two East and two West orbit velocity controlling thrusters and two North or two South orbit controlling thrusters. Each thruster string has an East and a West thruster mounted on Earth facing panel 12 and an East and a West thruster mounted on anti-Earth facing panel 14. Thus, thruster string A has thrusters 2A and 5A mounted on Earth facing panel 12 and thrusters 3A and 4A on anti-Earth facing panel 14. Thrusters 4A, 5A and 2B, 3B are generally eastward pointed adjacent East facing panel 16 and thrusters 2A, 3A and 4B, 5B are generally westward pointed adjacent West facing panel 18. South facing thrusters 6A and 7A are mounted on panels 12 and 14, respectively. The B string of thrusters are mounted in a similar manner, but thrusters 6B and 7B are North facing thrusters which provide South-direction thrust forces, as contrasted to thruster 6A and 7A which provide North directed thrust forces. The A string is the primary string for a North stationkeeping maneuver and the B string is the primary string for a South stationkeeping maneuver.
The 6 and 7 thrusters primarily provide roll torque, and the 2, 3, 4 and 5 thrusters primarily provide pitch and yaw torques.
Spacecraft 10 is assumed to be travelling in an easterly direction and its roll, pitch and yaw axes about the East, South and Earth directions are illustrated in FIG. 1. As is known to those skilled in the art, a geosynchronous spacecraft is often subjected to North or South stationkeeping orbit adjustments. Each such adjustment comprises a prescribed burn time of the 6A and 7A pair or 6B and 7B pair of thrusters to enable reorientation of the spacecraft's orbit in a North or South, respectively, direction.
If the torques exerted by thrusters 6B and 7B (for instance) have the identical magnitude but opposite sign, then the South stationkeeping maneuver will have negligible affect on the East/West orientation of spacecraft 10. However, this is often not the case and, due to variations in torque exerted by the firing thrusters, a torque effect is exerted on spacecraft 10. In response, an attitude control system within spacecraft 10 will compensate the unequal torques by causing the 6B or 7B thruster with the larger magnitude torque to pulse off, and to pulse on one or more East/West thrusters in a string. The firing of East/West attitude control thrusters (e.g., 2B-5B) results in force components being exerted along the East/West axis of the spacecraft's travel direction. Such forces can and do affect the spacecraft's orbit velocity and cause it to lose its longitudinal position relative to the Earth.
In FIG. 2, a plot is shown of pitch torque versus yaw torque. Vectors 20, 22, 24 and 26 illustrate the torque effects of the indicated thrusters. Thus, thruster 2A exerts both yaw and pitch component torques when it is pulsed. Similarly, thruster 2B (from string B) exerts similarly directed yaw and pitch torques when it is pulsed. But, thruster 2A and 2B are oppositely oriented so that thruster 2A exerts a force in the easterly direction and thruster 2B exerts a force in the westerly direction. Thus, when thruster 2A is pulsed, its yaw and pitch torque components create an increase in velocity in the easterly direction of spacecraft 10. Thruster 2B, by contrast, will exert a similar magnitude but oppositely directed affect on the velocity of spacecraft 10, and at the same time will exert similar yaw and pitch torques as thruster 2A.
It can be seen that by activating specific thrusters, pitch or yaw torques can be exerted on spacecraft 10 in such a manner as to counteract torques created by unbalanced North or South thrusters 6A, 7A or 6B, 7B. For instance, if a positive pitch torque is desired, thrusters 2A and 4A can be pulsed to provide a combined positive directed pitch torque. They also exert opposite and offsetting yaw torques. As can be seen from the positioning of thrusters 6A and 7A in FIG. 1, their simultaneous firing will impart a northerly velocity to spacecraft 10 and will primarily provide roll torque when one thruster is pulsed. Simultaneous firing of thrusters 6B and 7B provide a southerly directed thrust and also create, primarily, a roll torque when one thruster is pulsed.
The prior art describes a number of techniques for spacecraft attitude control, both through the use of coupled thrusters and through other techniques. U.S. Pat. No. 4,848,706 to Garg et al. describes a 3-axis control system wherein thruster locations cause significant cross-coupling torques. Attitude control signals are generated for spacecraft adjustment that take into consideration and automatically compensate for the cross-coupling torques.
U.S. Pat. No. 3,866,025 to Cavanagh describes a spacecraft attitude control system wherein orbit-adjust thrusters are used to perform both orbital inclination adjustment and roll or yaw attitude control. A reference system provides signals that enable an onboard computer to derive a shortest angle through which the total angular momentum vector of the spacecraft must be rotated to bring it into alignment with an orbit normal vector. In addition, an ideal torque is calculated to produce the rotation of the total angular momentum vector. A combination of orbit thrust adjusters is then chosen to produce a torque which best approximates the ideal torque.
U.S. Pat. No. 4,837,699 to Smay et al. describes a geosynchronous spacecraft control system that detects spin axis precession and develops thruster control signals to restore the spin axis to the desired attitude.
U.S. Pat. No. 4,767,084 to Chan et al. describes a stationkeeping maneuver for a 3-axis stabilized spacecraft in a geosynchronous orbit. When a momentum/reaction wheel in the spacecraft reaches saturation, thrusters are energized to desaturate the momentum/reaction wheel while simultaneously accomplishing a preselected compensation of the spacecraft in East/West position.
U.S. Pat. No. 3,984,071 to Fleming describes a control apparatus for damping roll/yaw motion within a control deadband of a spacecraft. When an error signal indicates an out-of-deadband condition, the control system initiates and controls the duration of operation of a thruster of proper orientation to cause the spacecraft's orientation to return to within deadband limits.
U.S. Pat. No. 3,944,172 to Becker describes another attitude control system wherein thrust bursts are employed to impart incremental momenta to the space vehicle when the vehicle moves beyond selected discrete deviations.
In the above attitude control systems, East/West orbit control is not performed simultaneously with a North or South stationkeeping maneuver.
It is an object of this invention to provide a method for East/West orbit control simultaneously with a North or South stationkeeping maneuver of a spacecraft, all while controlling spacecraft attitude.